Powertrain systems for use in vehicles are expected to operate over a wide range of speed and load conditions. One type of powertrain system includes an electrically variable transmission (EVT) which provides for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation.
An electrically variable transmission includes friction elements, referred to as clutches, which transfer torque from the internal combustion engine and electrical motors of the EVT to the final drive line. The clutches are typically hydraulically operated, and the torque capacity is based upon magnitude of applied clutch pressure. Higher hydraulic clutch pressure requires higher power input to the hydraulic pump. The difference between actual hydraulic pressure and required hydraulic pressure to meet the required torque capacity represents hydraulic power loss.
What is needed is a method and apparatus to dynamically control system main hydraulic clutch pressures of an electrically variable transmission, based upon required clutch capacity, as determined by output load of the transmission, to reduce hydraulic power loss.